1. Field of the Invention
The present invention relates in general to a steel sheet used as a material for automotive parts through welding, more specifically to a high tensile steel sheet excellent in high-speed arc weldability, capable of forming wide beads at an arc welding speed as high as 100 cm/min or above.
2. Description of the Related Art
Diverse welding processes are involved in assembly of automotive parts, and among them is a gas shielded arc welding particularly for an assembly of parts that need to have superior rigidity. This is because arc welding is generally known to increase joint strength, stabilize welding quality, and improve rigidity of parts. However, a shortcoming of the arc welding is that it takes a longer time than other processes such as cutting, pressing, and coating. Because of this, when arc welding is employed, production line speed is deteriorated, leading to an increase in production cost. Therefore, if the arc welding speed of steel sheets can be increased, the cost of production would be substantially reduced and it will be very meaningful industrially.
For instance, an invention in Japanese Patent Laid-Open No. S61-007089 tried to achieve high-speed gas shielded arc welding by incorporating a specific amount of Si and Mn into a steel wire for gas shielded arc welding. Unfortunately though, the cited invention is mainly directed to specify the composition of the wire, and does not necessarily take rigidity of a steel sheet into consideration. In addition, the cited invention unnecessarily defines the content of oxygen in the wire as 0.008% or more, so as to attain ripple uniformity of beads and reduction of tension on the beads.
Moreover, Japanese Patent Laid-Open No. 2000-167691 takes the composition of a steel sheet into account for arc welding of a high tensile steel sheet. The gist of the cited invention lies in a welding procedure of a high tensile steel sheet by specifying “Si content in the steel sheet (mass %)+Si content in the wire (mass %)≧1.5”. However, as the cited invention carries out such welding process at a speed of 30-60 cm/min, it is evident that the objective of the invention is not in accomplishment of a high speed arc welding process. Besides, the cited invention does not have any description about the influence of oxygen on arc welding.
In general, width of beads tends to decrease in a high-speed arc welding process. Thus, in order to put high-speed arc welding to practical use, it is necessary to ensure the width of beads to a certain extent particularly because groove position or wire position is often deviated or route gap is created during an actual arc welding process. However, none of the above-described cited inventions studied width of the bead during high speed arc welding.
Although there is a strong demand for light (thin) steel sheets in terms of enhancement of fuel efficiency in automobile industry, it should not outweigh the collision safety. To satisfy both sides, i.e., to obtain light and collision-safe steel sheets, more automobile manufacturers now use high tension steel sheets of reduced thickness. Such thin steel sheets, however, were easily melted away during high current arc welding so it was not easy to have a sufficient amount of welding at high current. Because of this, the welding bead width of a thin steel sheet has been narrowed and thus, it became necessary to ensure increased bead width. Furthermore, since high tensile steel, compared with mild steels, has poor stability in a pressing process, the route gap is easily increased. Therefore, there is a need to develop a way for increasing the bead width.